1. Field of the Invention
The present invention generally relates to back pressure valves which employ a spring loaded diaphragm to close a discharge port and more particularly to a valve having means for varying the controlled pressure in a flow line in response to external control system utilizing a conventional control actuator.
2. Description of the Prior Art
Back pressure valves in flow lines are conventionally in the form of a diaphragm valve spring biased into closed relationship to an exhaust port or orifice in which the center portion of the diaphragm or a seal or valve member positioned thereon engages and closes the exhaust orifice and the fluid to be controlled engages the diaphragm externally of the exhaust orifice and exerts pressure on the annular portion of the diaphragm between the exhaust orifice and the periphery of the diaphragm which is normally clamped between two components of the valve housing. When the pressure in the control fluid is sufficient to overcome the spring force, the diaphragm and the seal thereon which engages the exhaust orifice is lifted from the exhaust orifice thus allowing fluid to escape and lowering the pressure to the desired level. Conventionally, the controlled pressure is adjusted by means of a screw threaded member which bears axially against the outer end of a diaphragm spring with the screw threaded member extending through the housing and being accessible from the exterior of the valve. Once the screw threaded member has been adjusted, the back pressure valve will maintain relatively constant pressure in the controlled fluid. The primary variation in the controlled pressure is caused by the necessary compression of the spring as the diaphragm lifts to permit passage of fluid into the exhaust orifice. For this reason, such valves usually employ a relatively long compression spring in order to have a low spring constant so that it will maintain a more constant pressure.
The following U.S. patents illustrate back pressure valves generally of this type:
U.S. Pat. No. 1,924,896--Aug. 29, 1933
U.S. Pat. No. 2,041,906--May 26, 1936
U.S. Pat. No. 2,591,432--Apr. 1, 1952
U.S. Pat. No. 2,600,554--June 17, 1952
U.S. Pat. No. 2,669,249--Feb. 16, 1954
U.S. Pat. No. 2,745,254--May 15, 1956
U.S. Pat. No. 3,307,568--Mar. 7, 1967
U.S. Pat. No. 3,311,124--Mar. 28, 1967
U.S. Pat. No. 3,556,125--Jan. 19, 1971
U.S. Pat. No. 3,918,470--Nov. 11, 1975.
In many instances, it is desirable to vary the back pressure in a fluid flow line and in such instances, the above described conventional back pressure valves cannot be used since they do not enable a variation in the controlled pressure. For example, it is desirable to provide a variable back pressure in the water pressure delivered to the spray nozzles in an air washer in order to vary the effectiveness of the air washer and thereby produce air of controlled relative humidity. While controlled relative humidity has been accomplsihed by bypass dampers and other control measures, such arrangements are relatively complex and difficult to maintain in proper condition for accurate controlled relative humidity.